Search Results (50)

Accurate and precise spectral data for singly ionized silicon (Si ii) are important for numerous applications. In this work, we present high-resolution Fourier transform IR emission spectroscopy of Si ii, using spectra recorded with the 1 m FTS at the Kitt Peak National Observatory. A total of 18 lines were measured in the region 780 nm to 4000 nm, and an immediate comparison of our data with the NIST ASD shows that our measurements offer at least a tenfold improvement in precision. Full article

Stokes drift is the net displacement of ocean surface water particles caused by nonlinear surface waves. Its estimation typically relies on sea surface wave spectra, and truncation of the high-frequency spectral tail can significantly affect accuracy. This study uses directional wave spectrum data from the SWIM instrument onboard CFOSAT. By introducing a wind-speed-dependent parameterization scheme for the transition wavenumber (k_n) between the equilibrium and saturation ranges, as well as a cutoff wavenumber (k_m), we constructed a model to supplement the high-frequency tail of the wave spectrum combined with mask filtering to optimize spectrum reconstruction. The Stokes drift calculated with this model shows a better correlation (R = 0.699) with buoy observations than the widely used ERA5 reanalysis (R = 0.613). Analysis reveals pronounced regional differences in the contribution of high-frequency waves to surface Stokes drift, exceeding 80% in equatorial low-wind regions while dropping below 10% in the high-wind Southern Ocean due to enhanced breaking dissipation. The global Stokes drift distribution exhibits clear hemispheric asymmetry and seasonal evolution, with peak values (>0.12 m/s) in the Antarctic Circumpolar Current region. The proposed method provides a reliable, observation-based approach for improving global Stokes drift estimation, with direct implications for modelling ocean transport, Langmuir turbulence, and air–sea interactions. Full article

Research on hypersonic crossflow transition holds significant engineering and scientific importance. This paper investigates the impact of distributed roughness elements (DREs) on crossflow transition for a cone set at a 6° angle of attack, using experimental methods. The research was conducted in the Mach 6 wind tunnel, employing temperature-sensitive paint (TSP) as the measurement technology. Two types of nosetip were examined: a sharp nosetip with a radius of 0.1 mm and a blunt nosetip with a radius of 2.5 mm. The circumferential wavenumbers of the DREs on the nosetip included k = 35, k = 50, and k = 70. The results indicate that the nosetip with DREs at k = 50 has a more pronounced effect in promoting boundary layer transition to turbulence on the leeward side of the cone compared to the nosetips with DREs at k = 35 and k = 70. However, all three types of DREs exhibit similar effects on transition on the windward side. Additionally, the bluntness of the nosetip, at R = 2.5 mm, diminishes the effectiveness of DREs in promoting transition; however, the degree of diminished effectiveness varies with the circumferential azimuth. Full article

Using the results of numerical simulations and solar observations, this study shows that the transition from deterministic chaos to hard turbulence in the magnetic field generated by the emerging small-scale, near-surface (within the Sun’s outer 5–10% convection zone) solar MHD dynamos occurs through a randomization process. This randomization process has been described using the concept of distributed chaos, and the main parameter of distributed chaos $\beta$ has been employed to quantify the degree of randomization (the wavenumber spectrum characterising distributed chaos has a stretched exponential form $E\left(k\right)\propto exp-{(k/k_{\beta })}^{\beta }$). The dissipative (Loitsianskii and Birkhoff–Saffman integrals) and ideal (magnetic helicity) magnetohydrodynamic invariants govern the randomization process and determine the degree of randomization $0<\beta \le 1$ at various stages of the emerging MHD dynamos, directly or through Kolmogorov–Iroshnikov phenomenology (the magnetoinertial range of scales as a precursor of hard turbulence). Despite the considerable differences in the scales and physical parameters, the results of numerical simulations are in quantitative agreement with solar observations (magnetograms) within this framework. The Hall magnetohydrodynamic dynamo is also briefly discussed in this context. Full article

In recent years, domestic research on the ion substitution behavior and chromaticity of the mineral composition of “Ya’an Green” remains insufficient, while there is almost no relevant research on “Ya’an Green” abroad. In this study, X-ray powder diffraction (XRD), electron probe microanalysis (EPMA), infrared spectroscopy (IR), ultraviolet–visible spectroscopy (UV-Vis), and colorimetry were employed. The results indicate that the green and yellow matrices of “Ya’an Green” are primarily composed of muscovite, with rutile also present in the yellow matrix. In contrast, the white–green samples are mainly composed of quartz, with muscovite as a secondary mineral. Additionally, it was observed that the (004) crystal plane of muscovite exhibits a peak shift to lower 2θ angles, attributed to the substitution of Al³⁺ by ions with larger radii, such as Ba²⁺, Cr³⁺, and Fe²⁺, leading to an increase in unit cell parameters and a consequent shift in the peak to lower wavenumbers. The main elements of “Ya’an Green” are Al, Si, and K, with minor elements including Na, Fe, and Cr. Furthermore, Mg²⁺, Ca²⁺, Ti⁴⁺, Cr³⁺, and Fe²⁺ in the samples can substitute for Al³⁺ through isomorphic substitution. The infrared spectrum of muscovite in the ‘Ya’an Green’ sample shows three typical absorption peaks, 422 cm⁻¹ and 513 cm⁻¹ caused by Si-O bending vibration, 697 cm⁻¹ and 837 cm⁻¹ caused by Si-O-Al vibration, 948 cm⁻¹ caused by O-H bending vibration, and 3647 cm⁻¹ caused by O-H stretching vibration. The peak at 837 cm⁻¹ exhibits varying degrees of shift due to the substitution of Al³⁺ by ions with larger radii. The ultraviolet–visible spectra display two broad absorption bands at 422 nm and 615 nm, which are caused by Cr³⁺ transition, indicating that Cr is the chromogenic element responsible for the green color. A correlation was observed between the Cr³⁺ content and the hue angle h in “Ya’an Green” samples: the higher the Cr³⁺ content, the closer the hue angle is to 136^°, resulting in a darker green color, while lower Cr³⁺ content leads to a deviation from the dark green hue. This study establishes for the first time the correlation between the mineral composition of ‘Ya’an Green’ and its chromatic parameters and explores the linear relationship between its color and the number of color-causing elements and elemental substitution, which provide data support and theoretical models for the study of the color of seal stones. Full article

The interaction between terahertz (THz) photons and phonons of materials is crucial for the development of THz photonics. In this work, typical two-dimensional (2D) van der Waals (vdW) transition metal chalcogenide (TMD) layers and heterostructures are used in THz time-domain spectroscopy (TDS) measurements, low-wavenumber Raman spectroscopy measurements, calculation of 2D materials’ phonon spectra, and theoretical analysis of thermal responses. The TDS results reveal strong absorption of THz photons in the frequency range of 2.5–10 THz. The low-wavenumber Raman spectra show the phonon vibration characteristics and are used to establish phonon energy bands. We also set up a computational simulation model for thermal responses. The temperature increases and distributions in the individual layers and their heterostructures are calculated, showing that THz photon absorption results in significant increases in temperature and differences in the heterostructures. These give rise to interesting photothermal effects, including the Seebeck effect, resulting in voltages across the heterostructures. These findings provide valuable guidance for the potential optoelectronic application of the 2D vdW heterostructures. Full article

Spodumene (LiAlSi₂O₆) is a member of pyroxene-group minerals. It has the highest theoretical lithium abundance among all of the Li-bearing minerals. In the present work, in situ high-pressure Raman spectroscopic investigation of natural spodumene have been conducted up to 19.04 GPa. Unheated spodumene and spodumene recovered after heat treatments (up to 1000 °C) have also been analyzed by X-ray diffraction and infrared spectroscopy. The results indicate that spodumene, after the displacive C2/c–P2₁/c transformation triggered at ~3.2 GPa, remains stable at pressures up to 19 GPa at ambient temperature without undergoing decomposition, amorphization, or a second phase transition. The major OH bands of the spodumene samples are observed within the wavenumber range of 2580–3220 cm⁻¹, implying a strong hydrogen bond interaction. The water content of the spodumene is estimated to be 19–97 ppm wt. H₂O based on the integrated absorption area of the OH bands. The FTIR analysis of the spodumene samples recovered after heat treatments implies that spodumene can retain a significant amount of water (up to ~100 ppm H₂O by weight) under high-temperature conditions up to 1000 °C. This suggests that spodumene in subducted slabs is unlikely to undergo dehydration at temperatures below 1000 °C, and is therefore not expected to trigger partial melting. Thus, spodumene may serve as a key carrier for Li, transporting it into the deep mantle without releasing Li into melts during subduction. Full article

Urban heat islands (UHIs) constitute one of the most conspicuous anthropogenic impacts on local climates, characterized by elevated land surface temperatures in urban areas compared to surrounding rural regions. This study represents a novel and comprehensive effort to characterize the spectral signature of SUHI through the lens of the two-dimensional (2D) turbulence theory, with a particular focus on identifying energy cascade regimes and their climatic modulation. The theory of two-dimensional (2D) turbulence, first described by Kraichnan and Batchelor, predicts two distinct energy cascade regimes: an inverse energy cascade at larger scales (low wavenumbers) and a direct enstrophy cascade at smaller scales (high wavenumbers). These cascades can be detected and characterized through spatial power spectra analysis, offering a scale-dependent understanding of the SUHI phenomenon. Despite the theoretical appeal, empirical validation of the 2D turbulence hypothesis in urban thermal landscapes remains scarce. This study aims to fill this gap by analyzing the spatial power spectra of land surface temperatures across 14 cities representing diverse climatic zones, capturing varied urban morphologies, structures, and materials. We analyzed multi-decadal LST datasets to compute spatial power spectra across summer and winter seasons, identifying spectral breakpoints that separate large-scale energy retention from small-scale dissipative processes. The findings reveal systematic deviations from classical turbulence scaling laws, with spectral slopes before the breakpoint ranging from ~K^−1.6 to ~K^−2.7 in winter and ~K^−1.5 to ~K^−2.4 in summer, while post-breakpoint slopes steepened significantly to ~K^−3.5 to ~K^−4.6 in winter and ~K^−3.3 to ~K^−4.3 in summer. These deviations suggest that urban heat turbulence is modulated by anisotropic surface heterogeneities, mesoscale instabilities, and seasonally dependent energy dissipation mechanisms. Notably, desert and Mediterranean climates exhibited the most pronounced small-scale dissipation, whereas oceanic and humid subtropical cities showed more gradual spectral transitions, likely due to differences in moisture availability and convective mixing. These results underscore the necessity of incorporating turbulence theory into urban climate models to better capture the scale-dependent nature of urban heat exchange. The observed spectral breakpoints offer a diagnostic tool for identifying critical scales at which urban heat mitigation strategies—such as green infrastructure, optimized urban ventilation, and reflective materials—can be most effective. Furthermore, our findings highlight the importance of regional climatic context in shaping urban spectral energy distributions, necessitating climate-specific urban design interventions. By advancing our understanding of urban thermal turbulence, this research contributes to the broader discourse on sustainable urban development and resilience in a warming world. Full article

The ‘dynamo problem’ requires that the origin of the primarily dipole geomagnetic field be found. The source of the geomagnetic field lies within the outer core of the Earth, which contains a turbulent magnetofluid whose motion is described by the equations of magnetohydrodynamics (MHD). A mathematical model can be based on the approximate but essential features of the problem, i.e., a rotating spherical shell containing an incompressible turbulent magnetofluid that is either ideal or real but maintained in an equilibrium state. Galerkin methods use orthogonal function expansions to represent dynamical fields, with each orthogonal function individually satisfying imposed boundary conditions. These Galerkin methods transform the problem from a few partial differential equations in physical space into a huge number of coupled, non-linear ordinary differential equations in the phase space of expansion coefficients, creating a dynamical system. In the ideal case, using Dirichlet boundary conditions, equilibrium statistical mechanics has provided a solution to the problem. As has been presented elsewhere, the solution also has relevance to the non-ideal case. Here, we examine and compare Galerkin methods imposing Neumann or mixed boundary conditions, in addition to Dirichlet conditions. Any of these Galerkin methods produce a dynamical system representing MHD turbulence and the application of equilibrium statistical mechanics in the ideal case gives solutions of the dynamo problem that differ only slightly in their individual sets of wavenumbers. One set of boundary conditions, Neumann on the outer and Dirichlet on the inner surface, might seem appropriate for modeling the outer core as it allows for a non-zero radial component of the internal, turbulent magnetic field to emerge and form the geomagnetic field. However, this does not provide the necessary transition of a turbulent MHD energy spectrum to match that of the surface geomagnetic field. Instead, we conclude that the model with Dirichlet conditions on both the outer and the inner surfaces is the most appropriate because it provides for a correct transition of the magnetic field, through an electrically conducting mantle, from the Earth’s outer core to its surface, solving the dynamo problem. In addition, we consider how a Galerkin model velocity field can satisfy no-slip conditions on solid boundaries and conclude that some slight, kinetically driven compressibility must exist, and we show how this can be accomplished. Full article

This study investigates the effect of ultrasonic-assisted preparation on the structural and physicochemical properties of water caltrop starch-palmitic acid complexes as a function of ultrasound intensity and treatment time. All samples exhibited the characteristic birefringence of starch-lipid complexes under the polarized microscope, and flake-like and irregular structure under scanning electron microscope (SEM), indicating the formation of complexes through ultrasonic-assisted preparation. X-ray diffraction pattern further confirmed the transition from the original A-type structure for native starch to V-type structure for starch-lipid complexes, and the relative crystallinity of starch-lipid complexes increased as the ultrasound intensity and treatment time increased. Attenuated total reflectance-Fourier-transform infrared spectroscopy (ATR-FTIR) analysis indicated a decreasing trend in absorbance ratio at wavenumber of 1022 cm⁻¹/995 cm⁻¹, suggesting that the increase in the complex promoted the self-assembly within the short-range ordered structure, leading to the formation of bonds between the complexes. However, rapid-visco analysis (RVA) demonstrated that the viscosity generally decreased as the ultrasound intensity and treatment time increased, possibly due to the reduction in molecular weight by ultrasound. Differential scanning calorimetric (DSC) analysis revealed that the control starch-lipid complex without ultrasound treatment (US-0-0) exhibited two distinct endothermic peaks above 90 °C, representing Type I (95–105 °C) and Type II (110–120 °C) V-type complexes. However, ultrasound-treated samples showed only one peak around 95–105 °C and increased enthalpy (∆H), which was likely due to the breakdown of amylose and amylopectin, leading to more complex formation with palmitic acid, while the resulting shorter chains in the ultrasound-modified sample favor the formation of Type I complexes. Full article

Hybrid lead iodide perovskites are promising photovoltaic and light-emitting materials. Extant literature data on the key optoelectronic and luminescent properties of hybrid perovskites indicate that these properties are affected by electron–phonon coupling, the dynamics of the organic cations, and the degree of lattice distortion. We report temperature-dependent Raman studies of BA₂MAPb₂I₇ and BA₂MA₂Pb₃I₁₀ (BA = butylammonium; MA = methylammonium), which undergo two structural phase transitions. Raman data obtained in broad temperature (360–80 K) and wavenumber (1800–10 cm⁻¹) ranges show that ordering of BA⁺ cations triggers the higher temperature phase transition, whereas freezing of MA⁺ dynamics occurs below 200 K, leading to the onset of the low-temperature phase transition. This ordering is associated with significant deformation of the inorganic sublattice, as evidenced by changes observed in the lattice mode region. Our results show, therefore, that Raman spectroscopy is a very valuable tool for monitoring the separate dynamics of different organic cations in perovskites, comprising “perovskitizer” and interlayer cations. Full article

This paper reports for the first time on a new layered magnetic heterometallic erbium telluride EuErCuTe₃. Single crystals of the compound were obtained from the elements at 1120 K using CsI as a flux. The crystal structure of EuErCuTe₃ was solved in the space group Cmcm (a = 4.3086(3) Å, b = 14.3093(9) Å, and c = 11.1957(7) Å) with the KZrCuS₃ structure type. In the orthorhombic structure of erbium telluride, distorted octahedra ([ErTe₆]⁹⁻) form two-dimensional layers (${}_{\infty }{}^2\left\{{\left[\mathrm{E}\mathrm{r}{\left(\mathrm{T}\mathrm{e}1\right)}_{2/2}^e{\left(\mathrm{T}\mathrm{e}2\right)}_{4/2}^k\right]}^{-}\right\})$, while distorted tetrahedra ([CuTe₄]⁷⁻) form one-dimensionally connected substructures (${}_{\infty }{}^1\left\{{\left[\mathrm{C}\mathrm{u}{\left(\mathrm{T}\mathrm{e}1\right)}_{2/2}^e{\left(\mathrm{T}\mathrm{e}2\right)}_{2/1}^t\right]}^{5-}\right\}$) along the [100] direction. The distorted octahedra and tetrahedra form parallel two-dimensional layers (${}_{\infty }{}^2\left\{{\left[\mathrm{C}\mathrm{u}\mathrm{E}\mathrm{r}{\mathrm{T}\mathrm{e}}_3\right]}^{2-}\right\})$ between which Eu²⁺ ions are located in a trigonal-prismatic coordination environment (${\left[{\mathrm{E}\mathrm{u}\mathrm{T}\mathrm{e}}_6\right]}^{10-})$. The trigonal prisms are connected by faces, forming chains (${}_{\infty }{}^1\left\{{\left[{\mathrm{E}\mathrm{u}\left(\mathrm{T}\mathrm{e}1\right)}_{2/2}{\left(\mathrm{T}\mathrm{e}2\right)}_{4/2}\right]}^{2-}\right\}$) along the [100] direction. Regularities in the variations in structural parameters were established in the series of erbium chalcogenides (EuErCuCh₃ with Ch = S, Se, and Te) and tellurides (EuLnCuTe₃ with Ln = Gd, Er, and Lu). Ab-initio calculations of the crystal structure, phonon spectrum, and elastic properties of the compound EuErCuTe₃ were performed. The types and wavenumbers of fundamental modes were determined, and the involvement of ions in the IR and Raman modes was assessed. The experimental Raman spectra were interpreted. The telluride EuErCuTe₃ at temperatures below 4.2 K was ferrimagnetic, as were the sulfide and selenide derivatives (EuErCuCh₃ with Ch = S and Se). Its experimental magnetic characteristics were close to the calculated ones. The decrease in the magnetic phase transition temperature in the series of the erbium chalcogenides was discovered. Full article

When investigating complex atomic spectra, it may happen accidentally that two or even several transitions between different pairs of combining energy levels have nearly the same wavenumber, and the observed spectral lines are overlapping (blend situations). In such cases, investigations of hyperfine structures can be very helpful in the identification of the involved transitions. In this paper, two complicated blend situations within the spectra of lanthanide atoms (Praseodymium and Lanthanum) are discussed as examples. The experimental methods applied are optogalvanic and laser-induced fluorescence spectroscopy, combined with emission spectra gained via Fourier transform spectroscopy. It is shown that, in such cases, a combination of optogalvanic and laser-induced fluorescence detection is necessary to find all transitions contributing to the observed spectral signatures. Full article

We present a new perspective on the symmetries that govern the formation of large-scale structures across the Universe, particularly focusing on the transition from the seeds of galaxy clusters to the seeds of galaxies themselves. We address two main features of cosmological fluid dynamics pertaining to both the linear and non-linear regimes. The linear dynamics of cosmological perturbations within the Hubble horizon is characterized by the Jeans length, which separates stable configurations from unstable fluctuations due to the gravitational effect on sufficiently large (and therefore, massive enough) overdensities. On the other hand, the non-linear dynamics of the cosmological fluid is associated with a turbulent behavior once the Reynolds numbers reach a sufficiently high level. This turbulent regime leads to energy dissipation across smaller and smaller scales, resulting in a fractal distribution of eddies throughout physical space. The proposed scenario suggests that the spatial scale of eddy formation is associated with the Jeans length of various levels of fragmentation from an original large-scale structure. By focusing on the fragmentation of galaxy cluster seeds versus galaxy seeds, we arrived at a phenomenological law that links the ratio of the two structure densities to the number of galaxies in each cluster and to the Hausdorff number of the Universe matter distribution. Finally, we introduced a primordial magnetic field and studied its influence on the Jeans length dynamics. The resulting anisotropic behavior of the density contrast led us to infer that the main features of the turbulence could be reduced to a 2D Euler equation. Numerical simulations showed that the two lowest wavenumbers contained the major energy contribution of the spectrum. Full article

In this research, the wavy ice patterns that form due to the evolution of morphological perturbations on the water–ice phase transition interface in the presence of a fluid flow are studied. The mathematical model of heat transport from a relatively warm fluid to a cold wall includes the mechanism of convective–conductive heat transfer in liquid and small sinusoidal perturbations of the water–ice interface. The analytical solutions describing the main state with a flat phase interface as well as its small morphological perturbations are derived. Namely, the migration velocity of perturbations and the dispersion relation are found. We show that the amplification rate of morphological perturbations changes its sign with variation of the wavenumber. This confirms the existence of two different crystallization regimes with (i) a stable (flat) interfacial boundary and (ii) a wavy interfacial boundary. The maximum of the amplification rate representing the most dangerous (quickly growing) perturbations is found. The theory is in agreement with experimental data. Full article